MONOZYGOTIC TWINS

Commonly known as "identical twins" monozygotic twins are from a single egg that has divided after fertilization to create two embryos, consequently they share exactly the same genetic material and are of the same sex.

Monozygotic twins who have been separated in earliest infancy and raised apart have provided a classic research situation for social scientists because their genetic identity, yet different social experience, makes it possible to disentangle the separate effects of heredity and social environment.

The ban on human cloning in many countries worldwide is founded on an assumption that cloned children will be identical to each other and to their nuclear donor.

How identical would cloned children be? An understanding essential to the ethical debate - RG Edwards and HK Beard, Human Reproduction Journals - This paper explores the scientific basis for this assumption, considering both the principles and practice of cloning in animals and comparing genetic and epigenetic variation in potential human clones with that in monozygotic twins.

Cognitive and Personality Differences Between Identical Twins Following Skull Fractures - Michael J. Lyons and Adam P. Matheny, The University of North Carolina at Charlotte, University of Louisville School of Medicine, Journal of Pediatric Psychology 9(4) pp. 485-494, 1984 © 1984 Society of Pediatric Psychology 
The classical co-twin control method was used to evaluate the effects of age at the time of skull fracture on behavioral functioning several years later. The sample consisted of 13 pairs of male monozygotic twins. In each pair, one twin had suffered a noncompound skull fracture during one of the following two periods: between 12 and 36 months of age (five pairs) or between 36 and 48 months (eight pairs). All twins were given the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) at 6 years of age and their mothers rated the twins on a personality/temperament scale. When compared with their co-twins, the twins injured between 12 and 36 months had no cognitive deficits, but had higher scores on a factor denoting ratings of emotionality. The twins injured between 36 and 48 months, when compared with their co-twins, had significantly lower scores on four performance subtests of the WPPSI, but did not differ in ratings of personality/temperament. - All correspondence should be sent to Michael J. Lyons, who is now at the Psychology Department, Merrimack College, North Andover, MA 01845. - jpepsy.oxfordjournals.org/cgi/content/abstract/9/4/485

Female Monozygotic Twins with Selective Mutism-A Case Report. 
Journal of Developmental & Behavioral Pediatrics. 27(2):129-133, April 2006.
SHARKEY, L. 1; MC NICHOLAS, F. 2 
Abstract: Selective mutism is a rare social anxiety disorder characterized by a total lack of speech in certain specific situations despite the ability to speak in others. Both genetic and psychosocial factors are thought to be involved in its presentation, persistence, and response to treatment. This case report describes a case of young female monozygotic twins who presented with selective mutism and their treatment spanning a 2-year period. It highlights the strong genetic association along with environmental factors such as social isolation and consequences of maternal social phobia, all contributing to treatment resistance, despite an intensive multimodal biopsychosocial approach. General issues related to the difficulties in treating monozygotic twins are also addressed. - jrnldbp.com

Monozygotic twins discordant for major depression: a preliminary exploration of the role of environmental experiences in the aetiology and course of illness
KENNETH S. KENDLER and CHARLES O. GARDNER
Departments of Psychiatry and Human Genetics, Medical College of Virginia of Virginia Commonwealth University and Virginia Institute for Psychiatric and Behavioral Genetics, Richmond, VA, USA
Abstract: Background. Genetic effects upon behaviour are pervasive. To what extent are the many correlates of major depression (MD) due to individual-specific environmental experiences versus genetic factors correlated with risk for MD?
Methods. From a population-based twin registry, we identified 72 female monozygotic pairs discordant for a lifetime history of MD and compared the affected and unaffected members on a wide range of putative correlates of MD.
Results. The affected twin differed from her unaffected co-twin on many variables, eight of which were maximally discriminating: (i) maternal protectiveness; (ii) conflictual parent–child relationship; (iii) low optimism; (iv) current stressful life events; (v) financial difficulties and a history of (vi) phobia, (vii) nicotine dependence; and (viii) divorce. A cluster analysis suggested three ‘environmental pathways' to MD characterized by: (i) childhood vulnerability and anxiety; (ii) acting-out and demoralization; and (iii) interpersonal difficulties.
Conclusion. Important precursors and sequelae of MD originate in environmental experiences unique to the individual and are not mediated through genetic factors or family-of-origin effects. Such environmental factors cause pervasive differences in monozygotic twins discordant for MD, especially in the areas of interpersonal difficulties, psychopathology, social problems and self-concept. These findings should be interpreted in the context of possible retrospective recall bias and the difficulty of distinguishing risk factors from sequelae in co-twin–control studies. - journals.cambridge.org